Semiconductor device and method of manufacture thereof

ABSTRACT

There is provided a method by which lightly doped drain (LDD) regions can be formed easily and at good yields in source/drain regions in thin film transistors possessing gate electrodes covered with an oxide covering. A lightly doped drain (LDD) region is formed by introducing an impurity into an island-shaped silicon film in a self-aligning manner, with a gate electrode serving as a mask. First, low-concentration impurity regions are formed in the island-shaped silicon film by using rotation-tilt ion implantation to effect ion doping from an oblique direction relative to the substrate. Low-concentration impurity regions are also formed below the gate electrode at this time. After that, an impurity at a high concentration is introduced normally to the substrate, so forming high-concentration impurity regions. In the above process, a low-concentration impurity region remains below the gate electrode and constitutes a lightly doped drain region.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of manufacturing asemiconductor device possessing a lightly doped drain (LDD) region. Moreparticularly, the invention relates to a method of manufacturing a thinfilm transistor (TFT) possessing a gate electrode which is covered by anoxide film.

The invention further relates to a method of forming an insulated gatetype semiconductor device which is formed on an insulating surface andpossesses a silicon active layer in the form of a thin film and offorming an integrated circuit in which a large number of these devicesare formed. The semi-conductor devices of the invention can be used asthin film transistors or integrated circuits of such transistors in thedrive circuits of active matrices such as liquid crystal displays, etc.or image sensors, etc. or in SOI integrated circuits and conventionalintegrated circuits (microprocessors, microcontrollers, microcomputersand semiconductor memories, etc.). In the invention, ‘insulatingsurface’ does not just mean the surface of an insulating substrate butalso includes the surface of insulating films that are provided onsemiconductors or conductors.

2. Description of the Related Art

In recent years, the formation of insulated gate semiconductor devices(or MOSFETs) on insulating surfaces has been tried. Such formation ofsemiconductor integrated circuits on insulating surfaces is advantageousin respect of high-speed drive of circuits, since, as opposed toconventional semiconductor integrated circuits in which the speed ismainly governed by the capacitance (stray capacitance) of the wiring andthe substrate, this stray capacitance is not present on an insulatingsubstrate. A MOSFET which is formed on an insulating substrate in thismanner and possesses an active layer in the form of a thin film iscalled a thin film transistor (TFT). TFTs are essential for the purposeof raising the level of integration, and also for the purpose of formingintegrated circuits as multilayer circuits. For example, TFTs are usedas SRAM load transistors in semiconductor integrated circuits. It isalso known to form TFTs for the purpose of driving active matrix typeliquid crystal displays and image sensors, etc. In particular, becauseof the need for high-speed operation, crystalline silicon TFTs, withwhich mobility is higher, have recently been developed in place ofamorphous silicon TFTs, in which amorphous silicon is used for theactive layer.

If thin film transistors are to be used as drive elements in theindividual pixel regions of an active matrix type liquid crystaldisplay, it is necessary that the value of their off current be small.‘Off current’ is the current that flows between the source and draineven though the thin film transistor is in the ‘off state’. If the valueof this off current is large, the charge held for a pixel falls, and itbecomes impossible to maintain a screen display for a set time. Thereason why off current occurs is that the thin film transistorconstituting the active layer possesses a poly-crystalline structure ora microcrystalline structure.

For example, when an N-channel thin film transistor is in the off state,a negative voltage is imposed on the gate electrode. In this condition,the region of the channel-forming region which contacts the gateinsulation film is P-type. Therefore, a PN junction is formed betweenthe source and drain, and so hardly any current should flow. When,however, the active layer is constituted by a silicon film possessing apolycrystalline or a microcrystalline structure, migration of carriers(charges) via the crystal grain boundaries occurs, and this is the causeof off current.

An LDD (lightly doped drain) structure and an offset gate structure areknown as structures for making this off current small. These arestructures which are designed, mainly, to reduce the electric fieldstrength at and in the vicinity of the interface of the channel-formingregion and the drain region and thereby suppress migration of carriersvia the crystal grain boundaries in this region.

However, in the case of TFTs, unlike the case with known semiconductorintegrated circuit technology, there are still many problems that needto be solved, and there is the problem that it is difficult to producerequired LDD structures or offset gate structures. In particular, whenit is attempted to form a TFT on an insulating substrate such as a glasssubstrate, etc., there is the problem that, since the substrate becomeselectrostatically charged, reactive ion anisotropic etching fails tofunction properly and etching therefore becomes unstable, and there is,for example, the problem that it is difficult to form fine patterns withgood control.

FIG. 7 shows cross-sections of a typical LDD manufacturing process thathas been employed hitherto. First, a base film 702 is formed on asubstrate 701, and an active layer is formed with crystalline silicon703. Then, an insulation film 704 is formed with material such assilicon oxide, etc. on this active layer. (FIG. 7(A))

Next, a gate electrode 705 is formed with polycrystalline silicon (dopedwith an impurity such as phosphorus, etc.), or with tantalum, titaniumor aluminum, etc. Using this gate electrode as a mask, an impurityelement (phosphorus or boron) is introduced by ion doping or a similarmeans, thereby forming, in a self-aligning manner in the active layer703, lightly doped drain (LDD) regions 706 and 707 in which the dopantdose is small. The active layer region which is below the gate electrodeand into which an impurity has not been introduced comes to constitute achannel-forming region. The impurity with which doping has been effectedis then activated by a heat source such as a laser or a flashlamp, etc.(FIG. 7(B))

Next, an insulation film 708 of silicon oxide, etc. is formed by plasmaCVD, LPCVD or a similar means (FIG. 7(C)), and anisotropic etching ofthis film is effected to form a sidewall 709 adjacent the side surfaceof the gate electrode. (FIG. 7(D))

Then, the impurity element is introduced again, by ion doping or asimilar means, and, since the gate electrode 705 and sidewall 709 areused as a mask, regions (source/drain regions) 710 and 711 with quite ahigh impurity concentration are formed in a self-aligning manner in theactive layer 703. The doping impurity is then activated by a heat sourcesuch as a laser or a flashlamp, etc.

Finally, a layer insulator 712 is formed, contact holes are formed goingthrough the layer insulator to the source/drain regions, andwiring/electrodes 713 and 714 that connect to the source and drain areformed with metal material such as aluminum, etc. (FIG. 7(F))

Recently, products that require semiconductor integrated circuits to beformed on transparent insulating substrates have made an appearance.Examples are the drive circuits of optical devices such as liquidcrystal displays and image sensors. TFTs are also used in thesecircuits. These circuits are required to be formed with a large surfacearea, and a reduction in the temperature of the TFT manufactory processis therefore required. Also, in cases where a device with a large numberof terminals is on an insulating substrate and these terminals have tobe connected to a semiconductor integrated circuit, consideration hasbeen given to forming the actual semiconductor integrated circuit itselfor its first stage monolithically on the same insulating substrate inorder to reduce the packaging density.

Conventionally, a TFT is produced by annealing an amorphous,semi-amorphous or microcrystalline silicon film at a temperature of450-1200° C., to increase its crystallinity and improve it to agood-quality silicon film (ie, one with which mobility is sufficientlygreat), and using this as an active layer. There also exist amorphoussilicon TFTs using amorphous silicon for the active layer, but themobility in them is low, being 5 cm²/Vs, normally about 1 cm²/Vs, andconsiderations of operating speed, and also consideration of the factthat they do not permit production of P-channel TFTs mean that there areconsiderable restrictions on their use. Annealing at a temperature suchas noted above is necessary in order to produce a TFT in which themobility is 5 cm²/Vs. This annealing also makes it possible to produce aP-channel TFT (a PTFT).

Producing a high mobility TFT necessitates reducing the source/drainsheet resistance as well as that of the active layer. In particular, ifthe aim is to produce a TFT in which the field mobility exceeds 150cm²/Vs, the sheet resistance must be 200 Ω/square, and, in view of this,a method using silicides for portions corresponding to the source/drainhas been proposed.

FIG. 14 shows cross-sections of a typical currently devised TFTmanufacturing process in which a silicide is used in order to reduce thesheet resistance of the portion corresponding to the source/drainsection. First, a silicon active layer 1403 in the form of an island isformed on a substrate 1401. If required, a base film 1402 may be formedbetween the substrate and the active layer. Then, an insulation film1404 that functions as a gate insulation film is formed with materialsuch as silicon oxide, etc., on the active layer. (FIG. 14(A))

Next, a gate electrode 1405 is formed with poly-crystalline silicon(doped with an impurity such as phosphorus, etc. in order to lower theresistance), etc. Then, with this gate electrode as a mask, an impurityelement (phosphorus or boron) is introduced by ion doping or a similarmeans, and impurity regions 1406 are formed in a self-aligning manner inthe active layer 1403. The active layer region which is below the gateelectrode and into which an impurity has not been introduced comes toconstitute a channel-forming region. Then, the doping impurity isactivated by thermal annealing, laser annealing, flashlamp annealing,rapid thermal annealing or a similar means. (FIG. 14(B))

Next, an insulation film 1407 of silicon oxide, etc. is formed by plasmaCVD, LPCVD or a similar means (FIG. 14(C)), and a sidewall A 1408adjacent the side surface of the gate electrode is formed by anisotropicetching of this insulation film, by reactive ion etching or a similarmeans. (FIG. 14(D))

Then, a covering 1409 of a metal (eg, titanium, tungsten, molybdenum,platinum, or chromium, etc.) for forming a silicide over the wholesurface is formed. (FIG. 14(E))

This is followed by thermal annealing, laser annealing or a similarmeans to react the metal covering 1409 and the impurity regions 1406closely bonded thereto, and so form silicide regions 1410. At this time,the impurity region portions 1411 that are below the sidewall A 1408remain as impurity regions, since the metal covering 1409 is not formedon them. If silicon is used for the gate electrode, a silicide is alsoformed on the top surface of the gate electrode. On the other hand, themetal film deposited on the insulation film (silicon oxide, etc.) hardlyreacts at all, and so a portion of the metal covering 1409 becomes asilicide, and the other portion thereof remains unreacted.

If, at this time, the ratio of the etching rates of the metal covering1409 and its silicide is sufficiently great, it is possible to etch awayonly the unreacted metal covering. All the metals noted above aresuitable for this purpose, since their etching rates are greater thanthose of their silicides. (FIG. 14(F))

Finally, a layer insulator 1412 is formed, contact holes going throughthe layer insulator to the source/drain regions are formed, andwiring/electrodes 1413 connecting to the source and drain are formedwith metal material such as aluminum, etc. (FIG. 14(G))

In the element thus produced, the resistance of the silicide regions1410 is much smaller than that of ordinary doped silicon doped withphosphorus or boron, and it can be effectively ignored. Therefore, whatactually determines the source/drain sheet resistance is the width x ofan impurity region 1411 below the sidewall, and since this is verysmall, it is possible to produce a TFT in which the source/drain sheetresistance is satisfactorily small.

SUMMARY OF THE INVENTION

The above method directly follows the procedure of a conventional LDDmanufacturing process for semiconductor integrated circuits, and itcomprises a stage which, if unmodified, is difficult to implement inmanufacture of a TFT on a glass substrate, and stages which areundesirable in terms of productivity.

The difficulty lies in the formation of the sidewall. The thickness ofthe insulation films 708 and 1407 in FIG. 7 and FIG. 14 may be 0.5-2 μm,and since the thickness of the base film 702 or 1402 provided on thesubstrate is normally 1000-3000 Å, it often happens that a fall in theyield is caused due to the substrate being exposed because the baselayer is etched by error in the etching stage. Such faults must beavoided as much as possible, since substrates, apart from syntheticquartz, that are used in TFT manufacture contain many elements that areharmful to silicon semiconductors. Further, it is also difficult tofinish the sidewall to a uniform width. This is because fine control ofthe plasma in reactive ion etching (RIE) or similar plasma etching isdifficult, since the substrate surface, unlike that of siliconsubstrates used in semiconductor integrated circuits, is insulating.

Further, because of their high resistance, the width of LDDs must bekept as small as possible, and an advance to mass production isdifficult, because of the variability noted above, and the question ofhow to control a process for their self-alignment (ie, one in whichpositioning is effected without using a photolithographic method)constitutes a problem.

Another point is that, in the example shown in FIG. 14, when silicon isused for the gate electrode, a silicide is formed on the top surface ofthe gate electrode. However, when it is necessary to lower the gateelectrode/wiring resistance (which applies, eg, in cases where thecircuit size is large, and to liquid crystal displays, etc.), it is notalways advantageous to use silicon for the gate electrode and it wouldbe preferable to use metal material such as aluminum or titanium, etc.,but there is the problem that even if such a metal is made a compoundwith the metal film 1409, it is not possible to effect selectiveetching, since the etching rate of the resulting compound does notdiffer greatly from that of the metal film 1409.

If, for example, aluminum is used for the gate electrode 1405, andtitanium for the metal film 1409, titanium silicide is formed in theregions 1410. However, an aluminum/titanium alloy forms on the surfaceof the gate electrode. The titanium film can be etched by a mixedsolution consisting of a hydrogen peroxide aqueous solution and ammonia,but the aluminum gate electrode is also etched at the same time. Inother words, with the method of FIG. 14, the situation is that there isno choice but to use silicon or a silicide for the gate electrode, andthis is a considerable obstacle to the reduction of the gate electrode'sresistance.

It is accordingly the object of the present invention to provide amethod of forming a thin film transistor which resolves the aboveproblems and which simplifies the process. More specifically, it has asits object to resolve at least one of the following aspects.

(1) The manufacture, with good control, of thin film transistors with anLDD structure.

(2) The provision of a method of manufacturing, with excellentproductivity, thin film transistors with a small off current.

(3) The provision of thin film transistors whose off current is small.

(4) The provision of thin film transistors possessing characteristicsthat are not achieved with a conventional LDD structure or offset gatestructure.

It is an aspect of the structure of a 1st invention disclosed in thisspecification that,

in a TFT possessing a gate electrode covered by an oxide film, itcomprises

a step in which the gate electrode is used as a mask, and an impurity ata low dose is introduced obliquely into a substrate, thereby forminglow-concentration impurity regions in a self-aligning manner, and

a step in which an impurity at a high dose is introduced into thesubstrate from the vertical direction, thereby forminghigh-concentration impurity regions in a self-aligning manner,

wherein a lightly doped drain (LDD) region is formed below the oxidefilm.

A specific example of the above structure is shown in FIG. 3. The thinfilm transistor manufacturing stages shown in FIG. 3 are constituted asfollows. In step (B), a gate electrode 306 covered by an oxide film 307is formed. Then, using this gate electrode 306 and the oxide film 307around it as a mask, impurity ions (in this case phosphorus ions) areimplanted obliquely at a low dose in step (C), so forminglow-concentration impurity regions 308. What is referred to here as alow dose is preferably a dose of 1×10¹³-5×10¹⁴ cm⁻².

Then, in step (D), the impurity is introduced at a high dose from thevertical direction, so forming high-concentration impurity regions 309.What is referred to here as a high dose is preferably a dose of10¹⁴-5×10¹⁵ cm⁻².

As a result of execution of step (D), low-concentration impurity regionscan be formed in the active layer below the oxide film 307 that isaround the gate electrode. The portion that is present on the drain sideof these low-concentration impurity regions constitutes an LDD (lightlydoped drain) region.

It is an aspect of the structure of a 2nd invention disclosed in thisSpecification that

it comprises

an active layer,

a gate insulation film formed on this active layer, and

a gate electrode formed on this gate insulation film,

wherein an insulation layer in which the material constituting the gateelectrode has been oxidized is formed on the side surface of the gateelectrode,

and low-concentration impurity regions are formed in the active layerregion corresponding to the portion below this insulation film.

The structure shown in FIG. 3 can be cited as a specific example of theabove structure. The portion indicated by 304 in FIG. 3 is the activelayer, which is an important element constituting a thin filmtransistor. 305 is the gate insulation film. 306 is the gate electrode,and 307 is the insulation layer, which is produced by oxidizing the gateelectrode 306 in an anodic oxidation stage. 310 indicateslow-concentration impurity regions. The portion on the drain region sidefunctions as an LDD (lightly doped drain) region.

The arrangement in a 3rd invention is that

shield material provided around a gate electrode is taken as a mask, and

impurity ions are implanted obliquely into an active layer portion thatcorresponds to the portion below the shield material.

It is an aspect of a 4th invention that

it comprises

an active layer,

a gate insulation film formed on this active layer, and

a gate electrode formed on this gate insulation film,

wherein an oxide covering in which the material constituting the gateelectrode has been oxidized is formed on the side surface of the gateelectrode,

low-concentration impurity regions are formed in the active layer regionthat corresponds to the portion below this oxide film,

and a metal layer is formed on the outer surface of the portions of theactive layer region which constitute source and drain regions.

It is an aspect of the structure of a 5th invention that,

in a process for manufacturing a TFT possessing a gate electrode coveredwith an oxide covering, it comprises

a step in which the gate electrode is taken as a mask and an impurity isimplanted obliquely into the substrate,

and a step in which a metal layer is formed on the outer surface of theactive layer corresponding to the source and drain regions,

wherein, in the step in which the impurity is introduced, an impurityregion is formed in the active layer below the oxide covering.

A 6th invention is a method of manufacturing which comprises at leastthe following 9 steps, in the indicated order.

(1) A step in which a silicon region in the form of an island is formedon an insulating surface,

(2) a step in which an insulation film that functions as a gateinsulation film is formed on this silicon region,

(3) a step in which a gate electrode is formed on this insulation film,

(4) a step in which an anodic oxide is formed on the side surfaces andthe top surface of this gate electrode

(5) a step in which, with the gate electrode and anodic oxide taken as amask, the silicon region is irradiated, from an oblique direction, withaccelerated impurity ions, and impurity regions (source and drain) areformed in a self-aligning manner,

(6) a step in which, with the gate electrode and anodic oxide taken as amask, portions of the insulation film are removed, thereby exposing theupper surfaces of the impurity regions,

(7) a step in which a metal film is formed over the entire surface,

(8) a step in which the metal film and silicon are reacted, therebyforming silicide regions, and

(9) a step in which unreacted portions of the metal film are removed

A 7th invention is a method for manufacturing a semiconductor devicewhich comprises at least the following 9 steps, in the indicated order.

(1) A step in which a silicon region in the form of an island is formedon an insulating surface,

(2) a step in which an insulation film that functions as a gateinsulation film is formed on this silicon region,

(3) a step in which a gate electrode is formed on this insulation film,

(4) a step in which, with the gate electrode taken as a mask, thesilicon region is irradiated, from an oblique direction, withaccelerated impurity ions, and impurity regions (source and drain) areformed in a self-aligning manner,

(5) a step in which an anodic oxide is formed on the side surfaces andthe top surface of the gate electrode,

(6) a step in which, with the gate electrode taken as a mask, portionsof the insulation film are removed, thereby exposing the surfaces of theimpurity regions,

(7) a step in which a metal film is formed over the entire surface,

(8) a step in which this metal film and silicon are reacted, therebyforming silicide regions, and

(9) a step in which unreacted portions of the metal film are removed.

An 8th invention is a method of manufacturing a semiconductor devicewhich comprises at least the following 9 steps, in the indicated order,

(1) A step in which a silicon region in the form of an island is formedon an insulating surface,

(2) a step in which an insulation film that functions as a gateinsulation film is formed on this silicon region,

(3) a step in which a gate electrode is formed on this insulation film,

(4) a step in which an anodic oxide is formed on the side surfaces andthe top surface of this gate electrode,

(5) a step in which, with the gate electrode and anodic oxide taken as amask, portions of the insulation film are removed and the surfaces ofthe impurity regions are exposed,

(6) a step in which a metal film is formed over the entire surface,

(7) a step in which, with the gate electrode and anodic oxide taken as amask, the silicon region is irradiated, from a oblique direction, withaccelerated impurity ions, and impurity regions (source and drain) areformed in a self-aligning manner,

(8) a step in which the metal film and silicon are reacted, therebyforming silicide regions, and

(9) a step in which unreacted portions of the metal film are removed.

A 9th invention is a method of manufacturing a semiconductor devicewhich comprises at least the following 9 steps, in the indicated order.

(1) A step in which a silicon region in the form of an island is formedon an insulating surface,

(2) a step in which an insulation film that functions as a gateinsulation film is formed on this silicon region,

(3) a step in which a gate electrode is formed on this insulation film,

(4) a step in which an anodic oxide is formed on the side surfaces andthe top surface of this gate electrode,

(5) a step in which, with the gate electrode and anodic oxide taken as amask, portions of the insulation film are removed and the surfaces ofthe impurity regions are exposed,

(6) a step in which a metal film is formed over the entire surface,

(7) a step in which the metal film and silicon are reacted, therebyforming silicide regions,

(8) a step in which the unreacted portions of the metal film areremoved, and

(9) a step in which, with the gate electrode and anodic oxide taken as amask, the silicon region is irradiated, from an oblique direction, withaccelerated impurity ions and impurity regions (source and drain) areformed in a self-aligning manner.

It is a feature of the inventions disclosed in this Specification thatuse is made of an anodic oxide covering produced by anodic oxidation ofa gate electrode. Another feature is that formation of impurity regionsis effected by irradiating the substrate with accelerated impurity ionsfrom an oblique direction. The procedure employed in this process may bethat the substrate is rotated while held tilted relative to thedirection of the ion source (rotation-tilt ion implantation method).

The apparatus shown in FIG. 1 is used for this rotation-tilt ionimplantation. The apparatus shown in FIG. 1 comprises a chamber 101, asample holder (substrate holder) 102 and an anode 103 inside thischamber, a power supply 104 for supplying a high voltage to the anode103, and a grid electrode 105. The angle θ of the sample holder 102 canbe freely altered, so making it possible for ions to be injectedobliquely. Also, the sample holder is provided with a rotationmechanism, and it can be rotated during ion implantation.

A voltage of up to a maximum of 100 kV is imposed on the anode 103. As aresult of this high voltage, impurity ions 106 that are ionized by RFdischarge, etc. in the vicinity of the grid electrode 105 areaccelerated towards a substrate 107 (a sample) that is placed on thesample holder 102. Consequently, the accelerated impurity ions areimplanted in the substrate.

A conceptual representation of this rotation-tilt ion implantation isshown in FIG. 2. As shown in FIG. 2(A), a TFT on a substrate mounted onthe sample holder is held at an inclination θ relative to the ions withwhich doping is effected. The depth in which the impurity is introducedis determined by this inclination θ. In the invention, it is preferablethat this inclination θ be 30 degrees or more. Since this angle θ ismaintained, doping to as far as the portion below the gate electrode iseffected in the region indicated by 201.

In the region 202, however, since part of the region is in the shadow ofthe gate electrode, doping is effected only in the part which does notextend as far as the gate electrode. If, now, the sample holder isrotated 180 degrees, doping is also effected to as far as the portionunderneath the gate electrode in region 201, as illustrated in FIG.2(B). In this manner, ion doping at a low dose is effected. Thearrangement in this case may be that the sample holder is rotated 180degrees and doping is effected, but the same effect can be achieved moresimply if doping is effected while the sample holder is being rotated.In this Specification, rotation-tilt ion implantation is represented inthe manner shown in FIG. 2(C).

Effecting rotation-tilt ion implantation in this manner makes itpossible for an impurity layer doped to a set penetration distance to beformed easily and uniformly.

In particular, to form a low-concentration impurity region (constitutinga lightly doped drain (LDD) region), an impurity at a low dose isintroduced obliquely. After first effecting ion doping at a low dose,ion doping at a high dose is effected. In this case, the impurity isinjected from the direction that is normal to the TFT. With thearrangement made thus, no high-dose ion doping is effected and so alow-concentration impurity region is formed in the portion below thegate electrode in which low-dose ion doping was effected in thepreceding stage.

It is noted that, in formation of a low-concentration impurity region ina thin film transistor possessing a gate electrode covered by an oxidecovering film, by controlling the angle of incidence of the implantedimpurity, it is possible to form a low-concentration impurity regiononly in the portion that is below the anodic oxide, and it is alsopossible to form a low-concentration impurity region that overlaps thechannel-forming region.

It is thus both possible to form an LDD and possible to form an overlapLDD. What is meant here by ‘overlap LDD’ is a region which, like the LDDshown in FIG. 4, is the result of formation of an LDD region (indicatedas 415) to as far as underneath the gate electrode. In other words, inthe case of an overlap LDD, an LDD region is formed over a portion ofthe region which, conventionally, would constitute a channel-formingregion.

It is also a feature of the invention that a low-concentration impurityregion is formed by irradiating a substrate obliquely with impurityions. In the method illustrated in FIG. 1, accelerated impurity ions areobliquely incident, coming from a specific direction relative to thesubstrate, but since the substrate is rotated, the ultimate result,regardless of what the direction relative to the substrate is, isoblique injection of ions. In this process, the distance to which theimpurity is introduced is determined by the tilt angle θ and theacceleration voltage. In the invention, the tilt angle θ is preferably30° or more. Effecting rotation-tilt ion implantation in this mannermakes it possible for a doped low-concentration impurity region to beformed easily and uniformly to a set penetration distance.

In general, when irradiation with impurity ions is effected obliquely,the distance to which the ions penetrate is determined by the ionacceleration voltage (or acceleration energy) and the penetration angleθ. In the invention, since the penetration angle θ can be changed easilyand the width of the impurity region can therefore be controlled veryefficiently, the impurity region can be made such it overlaps the gateelectrode (be brought to an overlap state) or be so formed that it isdistant from the gate electrode (an offset state), as required. It istherefore possible to form an LDD only in the portion that is below theoxide, and it is also possible to form an overlap LDD that extends fromunderneath the gate electrode. Further, it is possible to make theimpurity region and source/drain relation that of an overlap state orthat of an offset state.

Since it is thus possible for the region constituting thelow-concentration impurity region to be formed with a good controlcharacteristic, it is possible to produce a thin film transistorpossessing required characteristics (in particular, the off currentcharacteristic).

Preferably, the metal covering that is for the purpose of forming asilicide in the invention is constituted by material that makes possiblethe formation of an ohmic or a close-to-ohmic, low-resistance contactwith a silicon semiconductor. Specifically, molybdenum (Mo), tungsten(W), platinum (Pt), chromium (Cr), titanium (Ti) and cobalt (Co) aresuitable. In practice in the invention, a silicide is produced byreacting at least one of these metals with silicon.

The anodic oxide plays an important role in connection with this in theinvention. If material such as aluminum, titanium or tantalum, etc. isused for the gate electrode, the anodic oxide produced reacts hardly atall with the metals noted above, and so the metal covering depositedthereon remains practically unreacted. Further, the anodic oxide acts asan etching stopper during etching of the metal covering.

Therefore, after formation of a silicide, the metal covering can beremoved without the gate electrode and other portions being etched andthe silicide of the portions corresponding to the source and drainremains.

In the invention, the choice of the gate electrode material isimportant, since, among other things, it determines the type of anodicoxide that will be formed. A pure metal such as aluminum, titanium ortantalum or an alloy of such a metal containing a small amount of anadditive (eg, an alloy in which 1-3% of silicon is added to aluminum)can be used for the gate electrode in the invention. It is noted that inthis specification, unless otherwise specified, the term aluminum istaken to mean not just pure aluminum but also material containing 10% ofan additive. The same also applies to titanium and other materials.

In the invention, use may be made of a gate electrode with asingle-layer structure using one of the above materials alone, or thegate electrode may be a multilayer structure in which these materialsare stacked in two or more layers. Examples are a two-layer structure inwhich titanium overlies aluminum, and a two-layer structure in whichaluminum overlies titanium. The thickness of each layer is determined bythe person practicing the invention in accordance with the requiredelement characteristics.

FIG. 10 will be taken as an example to describe the effects andadvantages of the above. The process of FIG. 10 is one that correspondsto the 6th invention described earlier. As shown in FIG. 10(A), a gateelectrode 1005 is formed on an active layer 1003 on a substrate 1001 onwhich a base film 1002 is formed, and an anodic oxide 1006 is formed onthe gate electrode's top surface and side surfaces. (FIG. 10(A))

Next, oblique irradiation with an impurity is effected to form impurityregions 1007. As a result of this, the impurity regions also go round tounderneath the anodic oxide 1006. (FIG. 10(B))

After that, an insulation film 1004 is etched, with the gate electrodeand anodic oxide film serving as a mask. This etching etches away theportions of the silicon oxide film 1004 other than the silicon oxidefilm 1008 portion thereof that lies below the gate electrode and theanodic oxide. (FIG. 10(C)). Further, a metal covering 1009 for forming asilicide is deposited over the whole surface. (FIG. 10(D))

Then, the metal covering and impurity regions are reacted, so formingsilicide regions 1011. However, the silicide reaction does not extend asfar as the impurity region 1010 portions that are below the anodicoxide, and these portions therefore remain as impurity regions. Further,since the metal covering formed on the anodic oxide remains in apractically unreacted state, the unreacted portions of the metal film1009 can be etched easily, and no etching of the gate electrode andother regions takes place.

In this manner, silicide regions 1011 and impurity regions 1010 areformed. Depending on how the silicide reaction progresses, the formationof the silicide may extend as far as the bottom of the active layer, asin FIG. 10(E), or may take place only on the surface of the activelayer, as in FIG. 10(F). Naturally, in the former case, the sheetresistance of the portion corresponding to the source/drain is small,and in the latter case, too, the resistance is sufficiently low. In bothcases, therefore, the source-drain sheet resistance is more or lessdetermined by the impurity region 1010 width x.

The silicide thickness, which, too, is related to the above, is selectedin accordance with the sheet resistance deemed necessary in the regioncorresponding to the source/drain. If it is required to achieve a sheetresistance of 10-100 Ω/square, this means that since the silicide'sresistivity is 0.1-1 mΩ·cm, the silicide thickness is suitably 100 Å-1μm.

In formation of a silicide in the invention, the silicide may beproduced by irradiating the metal film with a strong light such as alaser, etc. and causing reaction with the silicon semiconductor filmthat is underneath it. If a laser is used, a pulsed laser is preferable.With a continuous laser, there is a risk of peel-off due to expansion ofthe irradiated material caused by heat, and thermal damage of thesubstrate may occur, since the irradiation time is long.

By way of a pulsed laser, one may use an infrared laser such as anNd:YAG laser (Q-switched pulse oscillation being preferred), or, by wayof a 2nd harmonic thereof, a visible light laser, or various types ofultraviolet lasers using excimers such as KrF, XeCl and ArF, etc., butit is necessary to select a laser with a wavelength such that it is notreflected by the metal film when irradiation is effected from above themetal film. Basically, there is hardly any problem when the metal filmis very thin. Irradiation with laser light may also be effected from thesubstrate side. In this case, it is necessary to select laser light thatpasses through the silicon semiconductor film that is presentunderneath.

FIG. 11 shows a further development of the process of FIG. 10. First, abase film 1102, an active layer 1103, an insulation film 1104 thatfunctions as a gate insulation film, and a gate electrode 1105 that canbe anodically oxidized are formed on a substrate 1101, and anodicoxidation of the top surface and the side surfaces of the gate electrodeis effected, so producing an anodic oxide 1106. Then, with the gateelectrode and the anodic oxide as a mask, impurity regions 1107 areproduced in the active layer by oblique irradiation with impurity ions.The impurity concentration at this time is made lower than usual, andthe dose is set at, for example, 1×10¹³-5×10¹⁴ atoms/cm². (FIG. 11(A))

Next, impurity regions 1108 are formed by irradiation with ions of thesame conduction type from the generally vertical direction, with theimpurity concentration greater than that of the previously formedimpurity regions 1107. The dose in this case is suitably 1×10¹⁴-5×10¹⁵atoms/cm². As a result of this, low-concentration impurity regions 1109are formed below the anodic oxide film. (FIG. 11(B))

Then, with the gate electrode and anodic oxide as a mask, the insulationfilm 1104 is etched, and a metal covering 1110 is deposited over thewhole surface (FIG. 11(C))

Then, the metal covering and the impurity regions are reacted, andsilicide regions 1112 are formed. However, the silicide reaction doesnot extend as far as the impurity region 1109 portions that are belowthe anodic oxide, and these portions therefore remain as impurityregions. Since the metal covering formed on the anodic oxide remains ina practically unreacted state, the unreacted portions of the metalcovering 1110 can be etched easily, and no etching of the gate electrodeor other regions takes place.

In this manner, silicide regions 1112 are formed. Depending on how thesilicide reaction progresses, the formation of a silicide may extend asfar as the bottom of the active layer, as in FIG. 11(D), or may takeplace only on the surface of the active layer, as in FIG. 11(E). Thetype of formation should be selected in accordance with the requirementsof the person practicing the invention.

After that, an interlayer insulator 1113 is deposited, contact holes areformed in the silicide regions, and metal wiring-electrodes are formed.Once this is done, the TFT is completed.

In the example of FIG. 11, doping with an impurity at low concentrationwas effected for the source/drain regions. In an ordinary TFT, whendoping with an impurity at low concentration is effected in this manner,the field in the vicinity of the drain is eased, deterioration due tohot carrier injection is reduced and source-drain leakage current isalso reduced, but in a device as in, eg, FIG. 10 in which the impurityregions 1007 are low-concentration regions, because the impurityconcentration is low, the source-drain leakage current is liable toincrease when the drain voltage is high, since the NI junction (PIjunction in the case of a P-channel TFT) is thin and the distancebetween the silicide regions is short. High-concentration doping as inFIG. 11 is an effective way of preventing this.

FIG. 10 and FIG. 11 are drawings illustrating stages in the 6thinvention, and, needless to say, similar effects and advantages are alsoachieved in the other inventions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an apparatus for effecting ion doping.

FIGS. 2(A) to 2(C) illustrate the concept of tilt ion implantation.

FIGS. 3(A) to 3(E) show the stages of manufacture of the thin filmtransistor of a 1st example.

FIGS. 4(A) to 4(E) show the stages of manufacture of the thin filmtransistor of a 2nd example.

FIGS. 5(A) to 5(F) show the stages of manufacture of the thin filmtransistor of a 3rd example.

FIGS. 6(A) to 6(F) show the stages of manufacture of the thin filmtransistor of a 4th example.

FIGS. 7(A) to 7(F) show stages in a conventional method.

FIGS. 8(A) to 8(E) show the stages of manufacture of the thin filmtransistor of a 5th example.

FIGS. 9(A) to 9(F) show the stages of manufacture of the thin filmtransistor of a 6th example.

FIGS. 10(A) to 10(F) show the stages of manufacture of a thin filmtransistor of the invention.

FIGS. 11(A) to 11(F) show the stages of manufacture of another thin filmtransistor of the invention.

FIGS. 12(A) to 12(G) show the stages of manufacture of the thin filmtransistor of a 7th example.

FIGS. 13(A) to 13(F) show the stages of manufacture of the thin filmtransistor of an 8th example.

FIGS. 14(A) to 14(G) show the stages of manufacture of another thin filmtransistor according to a conventional method.

FIGS. 15(A) to 15(G) show the stages of manufacture of the thin filmtransistor of a 9th example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1

This example is shown in FIG. 3 and it is one in which an N-channel thinfilm transistor (TFT) possessing a lightly doped drain (LDD) region isformed by the invention.

First, silicon oxide was formed to 1000-5000 Å, eg, 4000 Å by a plasmaCVD process to constitute a base oxide film 302 on a substrate 301(Corning 7059, 100 mm×100 mm). This silicon oxide film serves to preventdiffusion of impurities from the glass substrate.

Then, an amorphous silicon layer 303 for forming an active layer wasformed to 300-1500 Å by a plasma CVD process or LPCVD process. In thiscase, it was formed to 500 Å by a plasma CVD process. This may befollowed by thermal annealing or laser annealing to effectcrystallization. There is no objection if a catalyst element such asnickel etc. is added at this time in order to promote crystallization.(FIG. 3(A))

Next, this amorphous silicon film was patterned to form a silicon film304 in the form of an island. This island-shaped silicon film 304constitutes the TFT's active layer. Then, a silicon oxide film with athickness of 200-1500 Å, 1000 Å in this case, was formed by a plasma CVDprocess, to constitute a gate insulation film 305.

After that a film of aluminum (containing 1 wt % of Si or 0.1-0.3 wt %of Sc) with a thickness of 1000 Å-3 μm, eg, 5000 Å, was formed by asputtering process and was patterned to form a gate electrode 306. Next,the substrate was immersed in 1-3% ethylene glycol solution of tartaricacid with a pH of approximately 7, and anodic oxidation was effected,with the aluminum electrode as the anode and with platinum as thecathode. In the anodic oxidation, the voltage was first raised to 220 V,with the current constant, and the process was completed by maintainingthis state for 1 hour. In this manner, a 2500 Å thick anodic oxide 307was formed. (FIG. 3(B))

After that, with the gate electrode serving as a mask, phosphorus wasintroduced as an impurity in a self-aligning manner into theisland-shaped silicon film 304 by an ion doping procedure. Phosphine(PH₃) was used as the doping gas. First, doping at a low dose waseffected by the rotation-tilt implantation procedure illustrated in FIG.2. The dose in this case was 1×10¹³-5×10¹⁴ atoms/cm² and theacceleration voltage was 10-90 kV, eg, a dose of 5×10¹³ atoms/cm² and anacceleration voltage of 80 kV. As a result, low-concentration impurityregions 308 were formed. (FIG. 3(C))

Next, high-dose ion doping was effected from the vertical direction. Thedose at this time is preferably 1-3 orders of magnitude greater than thedose in the preceding doping. In this example, it was 2×10¹⁵ atoms/cm²,40 times the preceding dose, and the acceleration voltage was 80 kV.This resulted in formation of high-concentration impurity regions 309.As a result of the above process, low-concentration impurity regionsremained below the gate electrode and lightly doped drain (LDD) regions310 were formed. (FIG. 3(D))

Further, the doped impurity regions 309 and 310 were activated byirradiation with a KrF excimer laser (wavelength 248 nm, pulse width 20nsec). This laser irradiation was not effected from the verticaldirection but was effected obliquely, as in the doping procedure at thetime of LDD formation. The laser energy density was suitably 200-400mJ/cm², and was preferably 250-300 mJ/cm². This stage may also beperformed by thermal annealing.

Next, a silicon oxide film was formed to a thickness of 3000 Å by aplasma CVD process to constitute an interlayer insulation film 311.Then, contact holes for the TFT's source and drain were formed byetching the interlayer insulation film 311 and gate insulation film 305.Then, an aluminum film was formed by a sputtering process and patternedto form source and drain electrodes 312. (FIG. 3(E))

The above process resulted in production of an N-channel TFT possessingan LDD. Formation of the TFT may be followed by the further step ofhydrogenation treatment at 200-400° C. for the purpose of activating theimpurity regions. In production of the LDD, control of the angle ofincidence in implantation of the impurity at the time of formation ofthe low-concentration impurity regions 308 makes it possible to form theLDD only under the anodic oxide 307, as in this example, or to make itan overlap LDD in which a low-concentration impurity region is formed asfar as underneath the gate electrode.

Example 2

This example is shown in FIG. 4, and it is one in which, using theinvention, an N-channel TFT possessing an overlap LDD and an N-channelTFT that does not possess such an LDD are formed on one and the samesubstrate.

First, similarly to Example 1, a silicon oxide film was formed to 3000 Åon a substrate 401 (Corning 7059) by a plasma CVD process to constitutea base oxide film 402. Then, an amorphous silicon film for forming anactive layer was formed to 500 Å by a plasma CVD process. After that,crystallization was effected by leaving this material in a reducingatmosphere at 550-600° C. for 8-24 hours. There is no objection if asmall amount of a catalyst element such as nickel, etc. for promotingcrystallization is added at this time. (FIG. 4(A))

Next, the resulting crystalline silicon film 403 was patterned to definesilicon films 404 and 405 in the form of islands. These island-shapedsilicon films constituted TFT active layers. Then, a 800 Å thick siliconoxide film was formed as a gate insulation film 406 by a plasma CVDprocess.

After that, a 6000 Å thick film of aluminum (containing 1 wt % of Si or0.1-0.3 wt % of Sc) was formed by a sputtering process, and waspatterned to define gate electrodes 407 and 408. Next, the substrate wasimmersed in a pH is approximately 7 ethylene glycol solution containing1-3% of tartaric acid, and anodic oxidation was effected, with thealuminum electrodes as anodes and with platinum as a cathode. For theanodic oxidation, the voltage was first raised to 220 V, with thecurrent constant, and the process was completed by maintaining thisstate for 1 hour. In this manner, 2500 Å thick anodic oxidation products409 and 410 were formed. (FIG. 4(B))

After that, with the gate electrode portions serving as masks,phosphorus was introduced in a self-aligning manner into theisland-shaped silicon films by an ion doping procedure. First, theregion that was to constitute an N-channel TFT without an LDD wascovered with a photoresist mask 411, and phosphorus was introduced intothe region that was to constitute an N-type TFT possessing an overlapLDD. In this process, doping at a low dose was effected by therotation-tilt ion implantation procedure illustrated in FIG. 2. Theangle of incidence for the ion implantation at this time was large, soas to effect formation of the low-concentration impurity region as faras underneath the gate electrode portion. The dose in this case was1×10¹³-5×10¹⁴ atoms/cm², and the acceleration voltage was 10-90 kV, eg,a dose of 4×10¹³ atoms/cm² and an acceleration voltage of 80 kV. Thisresulted in the formation of low-concentration impurity regions 412.(FIG. 4(C))

Next, the photoresist mask 411 covering the region that was toconstitute an N-channel TFT without an LDD was removed, and high-doseion doping was effected from the vertical direction. It is preferablethat the dose at this time be 1-3 orders of magnitude greater than thedose in the previously effected doping. In this example, the dose was2×15¹⁵ atoms/cm², 50 times the previous dose, and the accelerationvoltage was 80 kV. High-concentration impurity regions 413 and 414 wereformed as a result. The above process resulted in the formation ofN-type impurity regions (source/drain regions) with an overlap LDD 415,and N-type impurity regions without an LDD. (FIG. 4(D))

Further, in this example, the doped impurity regions 413, 414 and 415were activated by laser irradiation via the rear surface of thesubstrate. In this case, the laser light would be absorbed by thesubstrate and fail to reach the impurity regions if a KrF excimer laser(wavelength 248 nm, pulse width 20 nsec) were used, and therefore alaser with a different wavelength, in the form of an XeCl excimer laser(wavelength 308 nm, pulse width 30 nsec) or an XeF excimer laser(wavelength 353 nm, pulse width 40 nsec) should be used. An XeCl excimerlaser was used in this example. The laser energy density was suitably200-400 mJ/cm², and was preferably 250-350 mJ/cm². This stage may alsobe performed by thermal annealing.

Next, a silicon oxide film was formed to a thickness of 3000 Å as alayer insulation film 416 by a plasma CVD process. Then, contact holesfor the TFT sources and drains were formed by etching the layerinsulation film 416 and the gate insulation film 406. Then, an aluminumfilm was formed by a sputtering process, and was patterned to definesource/drain electrodes 417. (FIG. 4(E))

As the result of the above process, an N-channel TFT possessing anoverlap LDD and an N-channel TFT without an LDD were produced on one andthe same substrate. The TFT formation may be further followed byhydrogenation treatment at 200-400° C. for the purpose of activating theimpurity regions.

Example 3

This example is shown in FIG. 5, and it is one in which, using theinvention, a complementary circuit consisting of an N-channel TFTpossessing a lightly doped drain (LDD) and a P-channel TFT that does notpossess an LDD is formed.

First, similarly to Example 1, a silicon oxide film was formed to 3000 Åon a substrate 501 (Corning 7059) by a plasma CVD process to constitutea base oxide film 502. Then, an amorphous silicon film for forming anactive layer was formed to 500 Å by plasma CVD process. After that,crystallization was effected by leaving this material in a reducingatmosphere at 550-600° C. for 8-24 hours. There is no objection if asmall amount of a catalyst element such as nickel, etc. for promotingcrystallization is added at this time. (FIG. 5(A))

Next, the crystalline silicon film 503 was patterned, and silicon films504 and 505 in the form of islands were defined. These island-shapedsilicon films constituted TFT active layers. Then, a 800 Å thick siliconoxide film was formed as a gate insulation film 506 by a plasma CVDprocess.

After that, a 6000 Å thick film of aluminum (containing 1 wt % of Si or0.1-0.3 wt % of Sc) was formed by a sputtering process, and waspatterned to define gate electrodes 507 and 508. Next, the substrate wasimmersed in 1-3% ethylene glycol solution of tartaric acid with a pH ofapproximately 7, and anodic oxidation was effected, with the aluminumelectrodes as anodes and with platinum as a cathode. For the anodicoxidation, the voltage was first raised to 220 V, with the currentconstant, and the process was completed by maintaining this state for 1hour. In this manner, 2500 Å thick anodic oxidation products 509 and 510were formed. (FIG. 5(B))

After that, with the gate electrode portions serving as masks,phosphorus was introduced in a self-aligning manner into theisland-shaped silicon films by an ion doping procedure. First, theregion that was to constitute a P-channel TFT was covered with aphotoresist mask 511, and phosphorus was introduced into the region thatwas to constitute an N-type TFT possessing an overlap LDD. First, dopingat a low dose was effected by the rotation-tilt ion implantationprocedure illustrated in FIG. 2. The dose in this case was 1×10¹³-5×10¹⁴atoms/cm², and the acceleration voltage was 10-90 kV, eg, a dose of2×10¹³ atoms/cm² and an acceleration voltage of 80 kV. This resulted inthe formation of low-concentration impurity regions 512. (FIG. 5(C))

Next, high-dose ion doping was effected from the vertical direction. Itis preferable that the dose at this time be 1-3 orders of magnitudegreater than the dose in the preceding doping. In this example, the dosewas 1×15¹⁵ atoms/cm², 50 times the previous dose, and the accelerationvoltage was 80 kV. High-concentration impurity regions 513 were formedas a result. The above process resulted in the formation of N-typeimpurity regions (source/drain regions) with an LDD 514. (FIG. 5(D))

After that, the mask 511 covering the P-channel TFT region was removed,the region for constituting the N-channel TFT was covered with aphotoresist mask 515, and boron was introduced into the P-channel TFTregion. The dose in this case was 1×10¹³-5×10¹⁵ atoms/cm² and theacceleration voltage was 5-80 kV, eg, a dose of 1×10¹⁵ atoms/cm² and anacceleration voltage of 65 kV. This resulted in the formation of P-typeimpurity regions 516 (source/drain regions). (FIG. 5(E))

Further, the doped impurity regions 513, 514 and 516 were activated byirradiation with a KrF excimer laser (wavelength 248 nm, pulse width 20nsec). In this case, normal activation from the vertical direction waseffected. The laser energy density was suitably 200-400 mJ/cm², and waspreferably 250-300 mJ/cm². This stage may also be effected by thermalannealing.

Next, a silicon oxide film was formed to 3000 Å by plasma CVD process toconstitute a layer insulation film 517. Then, contact holes for the TFTsources and drains were formed by etching the layer insulation film 517and the gate insulation film 506. Then, an aluminum film was formed by asputtering process, and was patterned to define source/drain electrodes518.

The above steps produced a complementary circuit constituted by anN-channel TFT possessing an LDD and a P-channel TFT without an LDD.

Example 4

This example is shown in FIG. 6, and it is one which relates to amonolithic active matrix circuit. In this example, a complementarycircuit which was produced by the invention and was constituted by anN-channel TFT and a P-channel TFT possessing lightly doped drain (LDD)regions was used as a drive circuit, and a P-channel TFT that did notpossess an LDD was used as a matrix circuit switching element.

First, similarly to Example 1, a silicon oxide film was formed to athickness of 2000 Å on a substrate 601 (Corning 7059) by a plasma CVDprocess to constitute a base oxide film 602. Then an amorphous siliconfilm for constituting an active layer was formed by a plasma CVD processto a thickness of 500 Å. After that, crystallization was effected byleaving the material in a reducing atmosphere at 550-600° C. for 8-24hours.

Next, the amorphous silicon film was patterned to define silicon filmsin the form of islands. These island-shaped silicon films constitutedTFT active layers. Then, a 1000 Å thick silicon oxide film was formed asa gate insulation film 603 by a plasma CVD process.

After that, a 5000 Å thick aluminum film was formed by a sputteringprocess, and was patterned to define gate electrodes 604, 605 and 606.Next, the substrate was immersed in 1-3% ethylene glycol solution oftartaric acid with a pH of approximately 7, and anodic oxidation waseffected, with the aluminum electrodes as anodes and platinum as acathode, so producing 2000 Å thick anodic oxidation products 607, 608and 609.

After that, with the gate electrodes serving as masks, an impurity wasintroduced in a self-aligning manner into the island-shaped siliconoxide films by an ion doping procedure. First, the region that was toconstitute the P-channel TFT that did not possess an LDD and the regionthat was to constitute the drive circuit N-channel TFT were covered witha photoresist mask 610, and boron was introduced into the region thatwas to constitute the P-channel TFT of the drive circuit. First, dopingwas effected at a low dose by the rotation-tilt ion implantationprocedure illustrated in FIG. 2. The dose in this case was 1×10¹³-5×10¹⁴atoms/cm² and the acceleration voltage was 5-80 kV, eg, a dose of 3×10¹³atoms/cm² and an acceleration voltage of 60 kV. This resulted information of P-type low-concentration impurity regions 611 in the regionfor constituting the drive circuit P-channel type TFT. (FIG. 6(A))

Next, the photoresist mask covering the region for constituting theP-channel TFT without an LDD was removed, and the island-shaped siliconfilm was doped with boron ions from the vertical direction. It ispreferable that the dose in this case be 1-3 orders of magnitude greaterthan the dose in the previously effected doping. In this example, it was3×10¹⁵ atoms/cm², 100 times the previous dose, and the accelerationvoltage was 60 kV. This resulted in formation of high-concentrationimpurity regions 612 and 613. As a result of the above process, P-typeimpurity regions (source/drain regions) possessing an LDD 614 and P-typeimpurity regions (source/drain regions) without an LDD were formed.(FIG. 6(B))

Next, the photoresist mask 610 covering the region for constituting thedrive circuit's N-channel TFT was removed, and the P-channel TFT regionsinto which an impurity had been introduced earlier were covered withphotoresist masks 615.

After that, phosphorus was introduced into the region for constitutingthe drive circuit N-channel TFT. First, low-dose doping was effected bythe rotation-tilt ion implantation procedure illustrated in FIG. 2. Thedose in this case was 1×10¹³-5×10¹⁴ atoms/cm² and the acceleratorvoltage was 10-90 kV, eg, a dose of 3×10¹³ atoms/cm² and an accelerationvoltage of 70 kV. This resulted in formation of N-type low-concentrationimpurity regions 616 in the region constituting the drive circuit'sN-channel TFT. (FIG. 6(C))

Next, high-dose doping with phosphorus ions was effected from thevertical direction. It is preferable that the dose at this time be 1-3orders of magnitude greater than the dose in the previously effecteddoping. In this example, the dose was 3×10¹⁵ atoms/cm², 100 times theprevious dose, and the acceleration voltage was 70 kV. This resulted information of high-concentration impurity regions 617. As a result of theabove process, N-type impurity regions (source/drain regions) with anLDD 618 were formed. (FIG. 6(D))

Further, the doped impurity regions were activated by irradiation with aKrF excimer laser (wavelength 248 nm, pulse width 20 nsec). The laserenergy density was suitably 200-400 J/cm², and was preferably 250-300mJ/cm².

Next, a silicon oxide film was formed to a thickness of 3000 Å as alayer insulation film 619 by plasma CVD process. Then, contact holes forthe TFT sources and drains were formed by etching the layer insulationfilm 619 and gate insulation film 603. Then, an aluminum film was formedby a sputtering process, and was patterned to define source/drainelectrodes 620. (FIG. 6(E))

After that, a silicon nitride film was formed to a thickness of 3000 Åby a plasma CVD process to constitute a passivation film 621, and thisand the layer insulation film 619 and gate insulation film 603 wereetched to form a contact hole, and a pixel electrode 622 was formed by atransparent conductive film on the active matrix circuit TFT. (FIG.6(F))

The above process resulted in production of a monolithic active matrixcircuit in which a complementary circuit which was produced by theinvention and was constituted by an N-channel TFT and a P-channel TFTpossessing LDDs was used as the drive circuit and a P-channel TFTwithout an LDD was used as the active matrix circuit's switchingelement. It is noted that TFT formation may be followed by hydrogenationat 200-400° C. for the purpose of activating the impurity regions.

Example 5

This example is shown in FIG. 8. A special feature of the thin filmtransistor illustrated in this example is that, for the source/drainregion, use is made of a region in which a titanium nitride film isformed on a lightly doped drain (a region for which doping has beeneffected at a comparatively low concentration).

First, silicon oxide was formed to 1000-5000 Å, eg, 4000 Å by a plasmaCVD process to constitute a base oxide film 302 on a substrate 301. Thissilicon oxide film serves to prevent diffusion of impurities from theglass substrate.

Then, an amorphous silicon film 303 for constituting an active layer wasformed to 300-1500 Å by a plasma CVD process or an LPCVD process. Inthis case, it was formed to 700 Å by a plasma CVD process. This may befollowed by thermal annealing or laser annealing to effectcrystallization. There is no objection if a catalyst element such asnickel etc. is added at this time in order to promote crystallization ofthe amorphous silicon film.

Next, this amorphous silicon film was patterned to define a silicon film304 in the form of an island. This island-shaped silicon film 304constitutes the TFT's active layer. Then, a silicon oxide film with athickness of 200-1500 Å, 1000 Å in this case, was formed by plasma CVDprocess, to constitute a gate insulation film 305.

After that, a film of aluminum (containing 1 wt % of Si or 0.1-0.3 wt %of Sc) with a thickness of 1000 Å-3 μm, eg, 5000 Å, was formed by asputtering process and was patterned to define a gate electrode 306.Next, the substrate was immersed in 1-3% ethylene glycol solution oftartaric acid with a pH of approximately 7, and anodic oxidation waseffected, with the aluminum electrode as the anode and with platinum asthe cathode. In the anodic oxidation, the voltage was first raised to220 V, with the current constant, and the process was completed bymaintaining this state for 1 hour. In this manner, a 2500 Å thick anodicoxidation product 307 was formed. (FIG. 8(A))

After that, with the gate electrode portion as a mask, phosphorus wasintroduced as an impurity in a self-aligning manner into theisland-shaped silicon film 304 constituting an active layer by an iondoping or plasma doping procedure. Phosphine (PH₃) was used as thedoping gas. First, the doping was effected at a low dose by therotation-tilt procedure illustrated in FIG. 2. It is satisfactory forthis purpose if the dose is 1×10¹⁴-5×10¹⁴ atoms/cm² and the accelerationvoltage is 10-90 kV. In this case, the dose was 2×10¹⁴ atoms/cm² and anacceleration voltage of 80 kV. This resulted in formation oflow-concentration impurity regions 308. Further, a channel-formingregion 804 is formed in a self-aligning manner in this process. (FIG.8(B))

Next, the exposed gate insulation film 305 was removed, so giving thestate shown in FIG. 8(C). Further, a metal film 800 was formed to athickness of 50-500 Å by a sputtering process. In this case, a titaniumfilm was formed to a thickness of 200 Å by a sputtering process as themetal film 800. Instead of a titanium film, it is also possible to usemetal material such as nickel, molybdenum, tungsten, platinum orpalladium, etc.

Then, irradiation with laser light (eg, a KrF excimer laser) waseffected to activate the impurity in which doping had been effected andto recrystallize the low-concentration impurity regions, and, togetherwith this, to form a metal silicide layer 803 (a titanium silicide layerin this case) at and in the vicinity of the interface of the metal film800 and the active layer. If a KrF excimer laser is used, it issatisfactory if the laser light's irradiation density is 250-300 mJ/cm².Also, it is effective if the sample material is heated to 200-500° C.during the heating by laser light. Irradiation with intense light may beeffected instead of laser light irradiation. (FIG. 8(D))

After formation of the metal compound layer 803, the metal film 800 wasetched in an etching solution constituted by a 5:2:2 mixture of hydrogenperoxide, ammonia and water. Then, a silicon oxide film 311 was formedto a thickness of 6000 Å as a layer insulation film, and contact holesfor the TFT source and drain were formed in a hole opening stage. Then,an aluminum film was formed by a sputtering process, and was patternedto define source/drain electrodes 312. (FIG. 8(E))

The above process produced a TFT with an N-channel LDD. TFT formationmay be further followed by hydrogenation treatment at 200-400° C. forthe purpose of activating the impurity regions. The TFT shown in FIG.8(E) has a structure in which the source and drain regions have a metalsilicide film on a low-concentration impurity region, and in which alow-concentration impurity region that functions as an LDD region isformed between the channel-forming region and one or the other of thesource region and the drain region.

The source and drain regions 801, 802 in the structure shown in FIG.8(E) are lightly doped impurity regions, but since the metal silicidelayer 803 is formed on their surfaces, their resistance is low and theyfully perform their functions as source and drain regions. Thelow-concentration impurity regions 308 which do not have the metalsilicide layer 803 formed on them (and which coincide with the regionsindicated by 310) serve as regions for easing field concentrationadjacent the channel-forming region 804. (The one on the drain regionside being an LDD (lightly doped region)).

The TFT manufacturing process described in this example simplifies themanufacturing procedure and so makes it possible to improveproductivity, since impurity ion implantation is effected only once, andonly light doping is needed.

Example 6

The manufacturing process in this example of practice is shown in FIG.9. The special feature of the thin film transistor that is described inthis example is that a metal suicide layer for reducing the resistanceof the source/drain region and for improving the characteristics ofcontact between the source and drain regions and their electrodes isformed on the outer surface of the source/drain regions.

First, silicon oxide was formed to 1000-5000 Å, eg, 4000 Å by a plasmaCVD process to constitute a base oxide film 302 on a substrate 301. Thissilicon oxide film serves to prevent diffusion of impurities from theglass substrate.

Then, an amorphous silicon film for constituting an active layer wasformed to 300-1500 Å by a plasma CVD process or an LPCVD process. Inthis case, it was formed to 1000 Å by a plasma CVD process. This may befollowed by thermal annealing or laser annealing to effectcrystallization. There is no objection if a catalyst element such asnickel etc. is added at this time in order to promote crystallization.

Next, this amorphous silicon film was patterned to form a silicon film304 in the form of an island. This island-shaped silicon film 304constitutes the TFT's active layer. Then, a silicon oxide film with athickness of 100-1500 Å, 1000 Å in this case, was formed by a plasma CVDprocess, to constitute a gate insulation film 305.

After that a film of aluminum (containing 1 wt % of Si or 0.1-0.3 wt %of Sc) with a thickness of 1000 Å-3 μm, eg. 5000 Å, was formed by asputtering process and was patterned to form a gate electrode 306. Next,the substrate was immersed in ethylene glycol solution containing 1-3%of tartaric acid with a pH of approximately 7, and anodic oxidation waseffected, with the aluminum electrode as the anode and with platinum asthe cathode. In the anodic oxidation, the voltage was first raised to220 V, with the current constant, and the process was completed bymaintaining this state for 1 hour. In this manner, a 2500 Å thick anodicoxide 307 was formed. FIG. 9(A))

After that, with the gate electrode serving as a mask, phosphorus wasintroduced as an impurity in a self-aligning manner into theisland-shaped silicon film 304 by an ion doping or plasma dopingprocedure. Phosphine (PH₃) was used as the doping gas. First, doping ata low dose was effected by the rotation-tilt implantation procedureillustrated in FIG. 2. It is satisfactory if the dose in this case ismade 1×10¹⁴-5×10¹⁴ atoms/cm² and the acceleration voltage is 10-90 kV.Here, the dose was 2×10¹⁴ atoms/cm² and the acceleration voltage was 80kV. As a result, low-concentration impurity regions 308 were formed.Also, a channel-forming region 804 was formed in a self-aligning mannerin this stage. (FIG. 9(B))

Next, doping with phosphorus ions at a dose of 2×10¹⁵ cm⁻² was effectedby a plasma doping procedure. This doping was effected normally to thesubstrate. This stage resulted in the formation of a pair ofhigh-concentration impurity regions 309 serving as source and drainregions. (FIG. 9(C))

Next, the exposed gate insulation film 305 was removed, so producing thestate shown in FIG. 9(D). Further, a metal film 800 was formed by asputtering process to a thickness of 50-500 Å. In this case, a titaniumfilm was formed, by a sputtering process, to a thickness of 200 Å as themetal film 800. Metal material such as nickel, molybdenum, tungsten,platinum or palladium etc. can be used instead of a titanium film.

Then, irradiation with laser light (eg, a KrF excimer laser) waseffected, thereby activating the impurity with which doping had beeneffected and recrystallizing the low-concentration impurity regions,and, also, forming a metal silicide layer 803 (in this case, a titaniumsilicide layer) at and in the vicinity of the interface of the metalfilm 800 and the active layer. When a KrF excimer laser is used, it issatisfactory if the laser light irradiation density is 250-300 mJ/cm².Also, it is effective if the sample material is heated to 200-500° C.during laser heating. Also, intense light may be used instead of laserlight. (FIG. 9(E))

After formation of the metal silicide layer 803, the film metal film 800was etched with an etching solution constituted by a 5:2:2 mixture ofhydrogen peroxide, ammonia and water. A silicon oxide film 311 was thenformed to a thickness of 6000 Å as a layer insulation film. Further,contact holes for the TFT's source and drain were formed in ahole-opening stage. Then, an aluminum film was formed by a sputteringprocess, and was patterned to define source/drain electrodes 312. (FIG.9(F))

The above process resulted in production of a TFT possessing anN-channel LDD. Formation of the TFT may be further followed byhydrogenation at 200-400° C. for the purpose of activating the impurityregions. The TFT shown in FIG. 9(E) has a structure in which 310functions as a region that eases the field concentration between thechannel-forming region and the source and drain regions.

Example 7

The example is shown in FIG. 12, and it is one in which an N-channel TFTand a P-channel TFT are formed on one and the same substrate accordingto the invention. In this example, a suicide of the invention was usedonly for the N-channel TFT.

First, silicon oxide was formed to a 1000-5000 Å, eg, 1000 Å film by aplasma CVD process to constitute a base oxide film 1202 on a substrate1201 (Corning 7059, 100 mm×100 mm). This silicon oxide film serves toprevent diffusion of impurities from the glass substrate.

Then, an amorphous silicon film 1203 for defining an active layer wasformed to a thickness of 300-1500 Å, eg, 500 Å by a plasma CVD process.After that, crystallization was effected by leaving this material in areducing atmosphere at 550-600° C. for 8-24 hours. There is no objectionif a small amount of a catalyst element such as nickel, etc. forpromoting crystallization is added at this time. The crystallinity ofthe silicon film that had thus been crystallized was improved stillfurther by irradiating it with a KrF excimer laser (wavelength 248 nm).The laser energy density depends on the silicon film's crystallinity,but good results were achieved with 200-350 mJ/cm². The optimum energydensity also depended on the substrate temperature during laserirradiation. (FIG. 12(A))

Next, the resulting crystalline silicon film 1203 was patterned todefine silicon films 1204 and 1205 in the form of islands. Theseisland-shaped silicon films constituted TFT active layers. Then, a200-1500 Å, eg, 1200 Å thick silicon oxide film was formed as a gateinsulation film 1206 by a plasma CVD process.

After that, a 1000 Å-3 μm, eg, 6000 Å thick film of aluminum (containing1 wt % of Si or 0.1-0.3 wt % of Sc) was formed by a sputtering process,and was patterned to define gate electrodes 1207 and 1208. Next, thesubstrate was immersed in a 1-3% ethylene glycol solution of tartaricacid adjusted to pH is approximately 7.1 by ammonia, and anodicoxidation was effected, with the aluminum electrodes 1207 and 1208 asanodes and with platinum as a cathode, so forming anodic oxidationproducts 1209 and 1210. For the anodic oxidation, the voltage was firstraised to a specific voltage, with the current constant, and the processwas completed by maintaining this state for 1 hour. The specific voltageis determined in accordance with the thickness of the anodic oxidationproducts 1209 and 1210. In the above anodic oxidation process, thethickness of the anodic oxidation products is approximately proportionalto the maximum voltage imposed. In this manner, 600-2500 Å, eg, 1200 Åthick anodic oxidation products 1209 and 1210 were formed. The maximumimposed voltage in this process was 80-90 V. (FIG. 12(B))

Next, the region in which a P-channel TFT was to be formed was coveredwith a photoresist mask 1211, and the silicon oxide film in theN-channel TFT region was etched, with the gate electrode and anodicoxidation product serving as a mask. Only the region for the N-channelregion was etched, and the region for the P-channel region was notetched, since it was covered by the mask 1211.

A dry etching procedure was used for the etching, and CHF₃ was used asthe etching gas in this process. This is preferable since aluminum oxideconstituting the anodic oxidation product is hardly etched at all by dryetching, and so there is selective etching of only the silicon oxidefilm. Needless to say, a wet etching procedure, too, may be used, butif, for example, a hydrogen fluoride based etchant (eg, a mixed solutionof hydrofluoric acid and ammonium fluoride) is used, care is needed,since in this case the anodic oxidation product (aluminum oxide) can, beetched too. In this manner, the active layer of the N-channel TFT wasexposed.

Then, phosphorus was introduced at a low concentration into the regionfor forming the N-channel TFT. At this time, low-dose doping waseffected by the rotation-tilt ion implantation procedure illustrated inFIG. 1. The angle of incidence for the ion implantation at this time waslarge, so as to cause the low-concentration impurity region to be formedas far as underneath the gate electrode portion, and the accelerationvoltage, too, was large. The dose in this example was 1×10¹³-5×10¹⁴atoms/cm², and the acceleration voltage was 60-120 kV, eg, a dose of4×10¹³ atoms/cm² and an acceleration voltage of 110 kV. As a result ofthis, low-concentration N-type impurity regions 1212 were formed, andthese regions were so formed that they overlapped the gate electrode1207. (FIG. 12(C))

This stage may be followed by doping with phosphorus at a higher dosefrom a generally vertical direction, as illustrated in FIG. 11. Aftercompletion of doping, the photoresist mask 1211 covering the P-channelTFT region was removed, and a 5-50 nm thick titanium film 1009 wasformed by a sputtering procedure. (FIG. 12(D))

Then, annealing was effected for 1 hour at 300-550° C., eg, 350° C. As aresult of this, a silicide was formed in the portions where the titaniumfilm and the silicon film were in close contact, but as the titaniumfilm and the silicon oxide and aluminum oxide did not react, theseportions of the titanium film remained unreacted. The unreacted titaniumfilm was removed by an aqueous solution of hydrogen peroxide, water andammonia, eg, an etchant with the proportions hydrogenperoxide:ammonia:water=5:2:2. In this manner, silicide regions 1215corresponding to the N-channel TFT's source and drain were formed. Thoseportions of the low-concentration N-type impurity regions 1212 which hadnot become a silicide remained as regions 1214 in the gate electrodeportion.

Next, the region constituting the N-channel TFT was covered with aphotoresist mask 1216, and boron was introduced into the P-channel TFTregion. In this case, the impurity ions were injected generallyvertically, and the dose was 1×10¹³-5×10¹⁵ atoms/cm², and theacceleration voltage was 5-80 kV, eg, a dose of 1×10¹⁵ atoms/cm² and anacceleration voltage of 65 kV. This resulted in formation of P-typeimpurity regions 1217 (source/drain regions). (FIG. 12(E))

Then, with the gate electrode 1208 and anodic oxidation product 1210serving as a mask, the silicon oxide film 1206 of the P-channel regionwas again etched by dry etching procedure. This etching was effected inorder to make the N-channel TFT and P-channel TFT contact holes the samedepth.

After that, the impurity regions 1214 and 1217 were activated byirradiation with an XeCl excimer laser (wavelength 308 nm, pulse width30 nsec) from the rear surface. The laser energy density was suitably200-400 mJ/cm², and was preferably 250-300 mJ/cm². This stage may alsobe performed by thermal annealing. (FIG. 12(F))

The reason for using an XeCl excimer in this example is that if ashorter wavelength laser were used, the laser light would be absorbed bythe substrate (Corning 7059) and fail to reach the TFT's. A KrF excimerlaser (wavelength 248 nm) may be used if quartz is used for thesubstrate. Also, in cases in which, as in this example, borosilicateglass such as Corning 7059 is used, it is more effective if an XeFexcimer laser, which has a longer wavelength (353 nm), is used.

Next, a silicon oxide film was formed to a thickness of 3000 Å by aplasma CVD process to constitute a layer insulation film 1218. Then, thelayer insulation film 1218 was etched to form TFT source/drain contactholes. Then, an aluminum film was formed by sputtering and was patternedand etched to define source/drain electrodes 1219. (FIG. 12(G))

In this example, the N-channel TFT is a structure in which thelow-concentration impurity regions are caused to overlap the gateelectrode. This is because, especially in an N-channel TFT, there is aproblem that the conduction of the drain current is hindered by aparasitic P-type channel that is produced as the result of hot electronsbeing trapped in the gate insulation film (something which isparticularly marked when the maximum process temperature is ≦700° C.),but an effective measure to prevent formation of this parasitic channelis to make the impurity regions an overlap structure as described above.

Example 8

This example is shown in FIG. 13 and it is one that relates to amonolithic active matrix circuit in which an active matrix circuit thatis used in a liquid crystal display, etc. and a peripheral circuit fordriving it are formed on one and the same substrate. In this example, acomplementary circuit which was produced by using the invention andconsisted of an N-channel TFT and a P-channel TFT using a suicide wasused for the peripheral circuit, for which high-speed operation isdemanded, and a P-channel TFT with a low-concentration source and drainwas used as an active matrix circuit switching element. Use of theinvention for the peripheral circuit as in this example lowers the sheetresistance of (the region corresponding to) the source and drain and iseffective in respect of high-speed operation.

The manufacturing stages in this example will now be described withreference to FIG. 13. First, similarly to Example 1, and silicon oxidefilm was formed to 2000 Å on a substrate 1301 (Corning 7059) by a plasmaCVD process to constitute a base oxide film 1302. Then, an amorphoussilicon film for constituting an active layer was formed to 500 Å, andwas crystallized in the same way as in Example 1, and it was then etchedto define silicon regions 1303, 1304 and 1305 in the form of islands.These island-shaped silicon regions constituted TFT active layers.Further, a 1200 Å thick silicon oxide film 1306 was formed as a gateinsulation film by a plasma CVD process.

After that, gate electrodes 1307, 1308 and 1309 were formed by 5000 Åthick aluminum films. 1000 Å thick anodic oxide coverings were formed onthe top and side surfaces of the gate electrodes.

After that, the peripheral circuit P-channel TFT region and the activematrix circuit were covered with a photoresist mask 1310, and, with thisas a mask, the silicon oxide film 1306 of the peripheral circuitN-channel TFT was etched by a dry etching procedure.

Following this, an N-type impurity was introduced into the island-shapedsilicon region 1303 in a self-aligning manner, with the gate electrodeportion as a mask, by an ion doping procedure. In this example, dopingwas effected with phosphorus. A rotation-tilt ion doping procedure wasused for this doping. The dose was 1×10¹³-5×10¹⁴ atoms/cm², and theacceleration voltage was 60-120 kV, eg, a dose of 3×10¹⁴ atoms/cm² andan acceleration voltage of 120 kV. This resulted in the formation ofN-type impurity regions 1311 in the source and drain of the peripheralcircuit N-channel TFT. (FIG. 13(A))

Next, the photoresist mask 1310 covering the P-channel TFT region wasremoved, and photoresist masks 1312 were formed covering the peripheralcircuit N-channel TFT region and the active matrix circuit region. Then,using these masks, doping with a P-type impurity (boron in this case)was effected, again by rotation-tilt ion doping procedure, so formingP-type impurity regions 1313. The dose in this case was 1×10¹³-5×10¹⁵atoms/cm², and the acceleration voltage 40-90 kV, eg, a dose of 3×10¹⁴atoms/cm² and an acceleration voltage of 70 kV. (FIG. 13(B))

After that, using the masks 1312, the silicon oxide film 1306 of theP-channel TFT region was etched. As a result, therefore, the state wasone in which, in the peripheral circuit region, the silicon oxide film1306, apart from the silicon oxide films 1314 and 1315 below the gateelectrode portions, had been etched away, and the active layer in allthe portions corresponding to the TFT source/drain regions was exposed.After that, the photoresist masks 1312 covering the P-channel TFT regionwere removed, and a 5-50 nm thick titanium film 1316 was formed by asputtering procedure. (FIG. 13(C))

Then, silicide regions 1318 and 1319 were formed by reacting thetitanium film and the silicon film by annealing for 1 hour at 300-550°C., eg, 350° C. Unreacted titanium film was removed by an aqueoussolution of hydrogen peroxide, water and ammonia, eg, an etchant withthe proportions hydrogen peroxide:ammonia:water=5:2:2.

Consequently, a silicide was formed in the peripheral circuit TFTs, butno silicide was formed in the active matrix circuit, since its siliconactive layer was covered by the silicon oxide film 1306. (FIG. 13(D))

Those portions of the impurity regions 1311 and 1313 which had notbecome a silicide remained as regions 1320 and 1321 in the gateelectrode portions. After that doping with a P-type impurity (boron inthis case) at a low concentration was effected, so forminglow-concentration P-type impurity regions 1322. For the doping thistime, impurity ions were injected generally vertically. The dose in thiscase was 1×10¹²-5×10¹⁴ atoms/cm², and the acceleration voltage was 40-90kV, eg, a dose of 3×10¹³ atoms/cm² and an acceleration voltage of 65 kV.Since the actual dose in this doping was very small, there was almost noeffect on the peripheral circuit. (FIG. 13(E))

After that, the doped impurity regions were activated by irradiationwith an XeCl excimer laser (wavelength 353 nm) via the rear surface. Thelaser energy density was suitably 200-400 mJ/cm², and was preferably250-300 mJ/cm².

Next, a silicon oxide film was formed to a thickness of 3000 Å by aplasma CVD process to constitute a layer insulation film 1323. Then,contact holes for the TFT sources and drains were formed by etching thelayer insulation film 1323 and the gate insulation film 1306. Then, analuminum film was formed by sputtering, and was patterned to definesource/drain electrodes 1324.

After that, a silicon nitride film was formed to a thickness of 3000 Åby a plasma CVD process to constitute a passivation film 1325, and thisand the layer insulation film 1323 and gate insulation film 1306 wereetched to form a contact hole, and a pixel electrode 1326 wasconstituted by a transparent conductive film on the active matrixcircuit TFT. (FIG. 13(F))

The above stages produced a monolithic active matrix circuit in which acircuit which was produced using the invention and which consisted of anN-channel TFT and a P-channel TFT with a silicide in the regionscorresponding to the source and drain was used in the peripheralcircuit, and a P-channel TFT with a lightly doped source and drain wasused as an active matrix circuit switching element. The reason whysilicide regions were provided only in the peripheral circuit in thisexample is as described earlier. The reasons for making the source/drainregions in the active matrix circuit TFT lightly doped regions asdescribed above are that doing this achieves further reduction ofleakage current and, also, it has outstanding effects in respect ofreduction of deterioration caused by hot carriers on imposition of areverse bias voltage (a positive voltage in the case of a P-channel TFT)on the gate electrode.

Example 9

This example is also a monolithic active matrix circuit for a liquidcrystal display. The manufacturing stages in this example are shown inFIG. 15. A CMOS circuit was used for the peripheral circuit in thisexample, but, for simplicity only NTFTs are shown as the peripheralcircuit TFTs in FIG. 15. The peripheral logic circuit isrepresentatively shown on the left side in FIG. 15, and the matrixcircuit on the right.

A 2000 Å thick silicon oxide base film 202 was formed by a plasma CVDprocess on a glass substrate 1501. Monosilane (SiH₄) and nitrous oxide(N₂O) were used as the source gases in the plasma CVD process, and thesubstrate temperature at the time of film formation was 380-500° C., eg,430° C. The silicon oxide film 1502 thus formed was a hard film whoseetching rate was comparatively low. This is because nitrous oxide wasused as a source gas, and consequently the film became a silicon oxidefilm containing 1-10% of nitrogen. A typical etching rate in etching at23° C. by acetic-acid-buffered fluoric acid (ABHF) with hydrofluoricacid, ammonium fluoride and acetic acid in the proportions 1:50:50 was800-1100/minute.

After that a 500 Å thick amorphous silicon film was formed by a plasmaCVD process. Further, a very thin silicon oxide film (estimated to be40-100 Å) was formed on the surface of this amorphous silicon film byannealing for 1 hour at 550° C. in an oxidizing atmosphere. Then, a verythin film 45 of nickel acetate was formed by a spin coating procedure. A1-100 ppm nickel acetate aqueous solution was used for this procedure.The purpose of forming a thin silicon oxide film on the surface of theamorphous silicon film beforehand was to cause the aqueous solution tospread uniformly over the amorphous silicon surface.

Next, thermal annealing was effected for 4 hours at 550° C. in anitrogen atmosphere. Nickel acetate decomposes to give nickel at around400° C. and since the nickel acetate thin film was, effectively, inclose attachment to the amorphous silicon, this thermal annealing stageresulted in nickel penetrating into the amorphous silicon and causing itto crystallize, so producing a crystalline silicon region.

After that, the silicon film was irradiated with an XeCl excimer laser(wavelength 308 nm). In this example, the laser's energy density was250-300 mJ/cm². As a result, the crystallinity of the crystallinesilicon was improved still more.

Further, thermal annealing was effected again, in order to relieve thestrain caused by the stress of laser irradiation. In this example, thethermal anneal was performed for 4 hours at 550° C.

After that, the silicon film was etched, to form active layers 1503 and1504 in the form of islands. Then, a 1200 Å thick silicon oxide film1505 was formed as a gate insulation film by a sputtering procedure.

Further, a 4000 Å thick aluminum film (containing 0.2-0.3 wt % ofscandium) was formed by a sputtering procedure. Then, by anodicoxidation of the surface of this film, a 100-300 Å thick aluminum oxidefilm (not shown in the drawing) was formed. The presence of the aluminumoxide film made adhesion with a photoresist good and, throughsuppression of leakage of current from the photoresist, it was effectivein causing a porous anodic oxide to be formed only on the side surfacein a subsequent anodic oxidation stage.

Then, a photoresist (eg, OFPR800/30 cp manufactured by Tokyo Oka) wasformed by a spin coating procedure, and was patterned and etched todefine gate electrodes 1509 and 1511 and a gate line 1510. Theperipheral circuit gate electrode 1509 and gate line 1510, and matrixcircuit gate electrode 1511 were electrically insulated. The photoresistmasks 1506, 1507 and 1508 that were used in the etching were left asthey were. (FIG. 15(A)

Next, with the photoresist masks still attached, porous anodic oxidationwas effected by passing current through the gate line 1510 (and hencethe gate electrode 1511), so forming on the side surfaces of this gateline and gate electrode porous anodic oxidation products 1512 and 1513.The anodic oxidation was effected using a 3-20% acidic aqueous solutionof citric acid, oxalic acid, phosphoric acid, chromic acid or sulfuricacid, etc. It is satisfactory if a constant-current voltage of 10-30 Vis imposed on the gate electrode.

In this example, anodic oxidation was effected for 20-40 minutes in apH=0.9-1.0 oxalic acid solution (30° C.), with the voltage 10 V. Theanodic oxide thickness was controlled by means of the anodic oxidationtime. When anodic oxidation is effected in an acidic solution such asthe above, a porous anodic oxide is formed. In this example, thethickness of the porous anodic oxidation products 1512 and 1513 was3000-10000 Å, eg, 5000 Å. (FIG. 15(B))

Next, the photoresist masks were peeled off, and barrier type anodicoxidation was effected by passing current through the gate line 1510, soforming fine barrier anodic oxide coverings 1514 and 1515 to a thicknessof 1200 Å on the side surfaces and top surfaces of the gate line andgate electrode. (FIG. 15(C))

Next, with the porous anodic oxidation products 1512 and 1513 as masks,the silicon oxide film 1505 was etched by a dry etching procedure, soresulting in formation of gate insulation films 1517 and 1518. Thisetching may be effected either in isotropic etching plasma mode or inanisotropic etching reactive ion etching mode. However, it is importantthat the silicon and silicon oxide selection ratio be made sufficientlygreat to avoid excessive etching of the active layer. For example, ifCF₄ is used as the etching gas, the anodic oxidation product is notetched, and only the silicon oxide film 1505 is etched. Also, thesilicon oxide films 1517 and 1518 below the porous anodic oxidationproducts 1512 and 1513 remained unetched. (FIG. 15(D))

Further, a mixed solution (aluminum mixed acid) of phosphoric acid,acetic acid and nitric acid was used to etch only the porous anodicoxidation products. The porous anodic oxidation products were etched bythe aluminum mixed acid, but the barrier anodic oxide coverings 1514 and1515 were hardly etched at all by it. However, since aluminum was beingetched, the peripheral circuit portion was masked with a photoresist, inorder to protect its gate electrodes. Therefore, although aphotolithography stage is added, there is the advantage that the levelof integration of the peripheral circuit portion can be raised.

Then, using the gate insulation film, impurities (phosphorus and boron;although the drawing shows only an NMOS, in actual fact boron doping wasalso effected) were introduced into the active layer by an ion dopingprocedure. To take phosphorus doping as an example, first, phosphorusions were injected from the vertical direction at the comparatively highdose of 5×10¹⁴-5×10¹⁵ atoms/cm² and the comparatively low accelerationvoltage of 10-30 keV. Since the acceleration voltage was low in thisprocess, the depth of ion penetration was shallow, and the phosphoruswas implanted mainly in the regions 1519 and 1520 where silicon wasexposed.

Next, using the apparatus shown in FIG. 1, phosphorus ions wereimplanted at a comparatively low dose by the rotation-tilt ionimplantation procedure illustrated in FIG. 2. In this case, theacceleration voltage was comparatively high, and was 60-95 keV, and thedose was 1×10¹²-1×10¹⁴ atoms/cm². Because the acceleration voltage washigh this time, the ions penetrated deeply, and phosphorus was alsoimplanted in the regions 1521 covered by the gate insulation film.

As a result, there were formed regions 1519 and 1520 doped withphosphorus at a high concentration and regions 1521 doped withphosphorus at a low concentration. In other words, a so-called doubledrain structure was produced for the pixel TFT. The same procedureshould also be employed for boron. After that, irradiation with a KrFexcimer laser (wavelength 248 nm, pulse width 20 nsec) was effected inorder to improve the crystallinity of the portions whose crystallinityhad deteriorated because of the formation of the above-noted impurityregions. The laser energy density was suitably 200-400 mJ/cm², and waspreferably 250-300 mJ/cm². (FIG. 15(E))

A multilayer film 1522 consisting of a 200 Å thick silicon oxide filmand a 4000 Å thick silicon nitride film was deposited as a 1st layerinsulator by a plasma CVD process, and was etched by a dry etchingprocedure to form contact holes 1523, 1524, 1525, 1526 and 1527. (FIG.15(F))

Then, a titanium 500 Å/aluminum 4000 Å/titanium 500 Å 3-layer metal filmwas deposited by a sputtering procedure, and was etched to formelectrode/wiring 1528, 1529, 1530 and 1531.

Further, a 2000 Å thick silicon oxide film 1532 was deposited as a 2ndlayer insulator by a plasma CVD process, a contact hole to the electrode1531 on the pixel TFT's drain side was formed, and a pixel electrode1533 was formed with ITO. In this manner, formation of a monolithicactive matrix circuit was completed. (FIG. 15(G))

Conventionally, an LDD region is formed by forming a sidewall adjacentthe side surface of a gate electrode after formation of alow-concentration impurity region, but with this procedure, sinceformation of a sidewall is difficult and takes time and effort, theyield is low and there are problems in terms of productivity. In thepresent invention, the arrangement is made such that formation of asidewall is not necessary, but a TFT possessing an LDD can easily beformed, simply by introducing an impurity at a low concentration from anoblique direction and an impurity at a high concentration from thevertical direction in ion doping stages in ordinary TFT manufacture, andthe invention is therefore advantageous from the point of view ofimproving yields.

Further, by controlling the angle of incidence for impurity injection atthe time of formation of a low-concentration impurity region in a TFTpossessing a gate electrode covered by an oxide covering, it is possibleto produce a TFT with a structure which has an LDD only under the oxidecovering, or a TFT with an overlap LDD structure.

Further, since the greater portion of the impurity region correspondingto the source/drain region can be made as a suicide, the sheetresistance of this region can be lowered. It is noted that, althoughpractice of the invention necessitates the addition of an anodicoxidation stage, the anodic oxide coverings produced give the advantagethat the insulation between wiring is reinforced. Also, in theinvention, controllability of N-type or P-type impurity regions can bemade very much better than it is with conventional methods, and so theprobability of occurrence of faulty parts can be greatly reduced.

Thus, use of the invention disclosed in this Specification makes itpossible for thin film transistors possessing required characteristicsto be produced with good productivity and at high yields.

1. A method for manufacturing a semiconductor device comprising: forminga semiconductor region; forming a gate insulation film over thesemiconductor region; forming a gate electrode over the gate insulationfilm, wherein the gate electrode comprises metal material; forming anoxide layer over and in contact with the gate electrode; doping animpurity element into the semiconductor region to form alow-concentration impurity region in contact with a channel-formingregion; after doping, removing a portion of the gate insulation film soas to expose at least a side surface of the semiconductor region; afterdoping, activating the impurity element by heating; forming a metal filmso as to cover at least the side surface of the semiconductor region anda top surface of the gate electrode, wherein the metal film is providedover the gate electrode with the oxide layer interposed therebetween;and forming a metal silicide layer over the semiconductor region,wherein a part of the low-concentration impurity region overlaps withthe gate electrode.
 2. A method for manufacturing a semiconductor deviceaccording to claim 1, wherein the low-concentration impurity region isformed by doping the impurity element in an oblique direction withrespect to a top surface of the semiconductor region.
 3. A method formanufacturing a semiconductor device according to claim 1, wherein themetal of the metal film is titanium.
 4. A method for manufacturing asemiconductor device according to claim 1, wherein the impurity elementis phosphorus.
 5. A method for manufacturing a semiconductor deviceaccording to claim 1, wherein a dose in forming the low-concentrationimpurity region is from 1×10¹³ to 5×10¹⁴ cm⁻².
 6. A method formanufacturing a semiconductor device according to claim 1, wherein themetal of the metal film is one of nickel and platinum.
 7. A method formanufacturing a semiconductor device according to claim 1, furthercomprising: after forming the metal silicide layer, forming aninterlayer insulation film over the gate electrode, the interlayerinsulation film being in contact with the gate insulation film.
 8. Amethod for manufacturing a semiconductor device according to claim 1,wherein the metal film is formed so as to be in contact with the gateinsulation film.
 9. A method for manufacturing a semiconductor devicecomprising: forming a semiconductor region; forming a gate insulationfilm over the semiconductor region; forming a gate electrode over thegate insulation film, wherein the gate electrode comprises metalmaterial; forming an oxide layer over and in contact with the gateelectrode; first doping an impurity element into the semiconductorregion to form a low-concentration impurity region in contact with achannel-forming region; after first doping, second doping an impurityelement into the semiconductor region to form a high-concentrationimpurity region in contact with the low-concentration impurity region;after second doping, removing a portion of the gate insulation film soas to expose at least a side surface of the semiconductor region;forming a metal film so as to cover at least the side surface of thesemiconductor region and a top surface of the gate electrode, whereinthe metal film is provided over the gate electrode with the oxide layerinterposed therebetween; and forming a metal silicide layer over thesemiconductor region.
 10. A method for manufacturing a semiconductordevice according to claim 9, wherein the low-concentration impurityregion is formed by doping the impurity element in an oblique directionwith respect to a top surface of the semiconductor region.
 11. A methodfor manufacturing a semiconductor device according to claim 9, whereinthe metal of the metal film is titanium.
 12. A method for manufacturinga semiconductor device according to claim 9, wherein the impurityelement is phosphorus.
 13. A method for manufacturing a semiconductordevice according to claim 9, wherein a dose in forming thelow-concentration impurity region is from 1×10¹³ to 5×10¹⁴ cm⁻².
 14. Amethod for manufacturing a semiconductor device according to claim 9,wherein a dose in forming the high-concentration impurity region is from1×10¹⁴ to 5×10¹⁵ cm⁻².
 15. A method for manufacturing a semiconductordevice according to claim 9, wherein the metal of the metal film is oneof nickel and platinum.
 16. A method for manufacturing a semiconductordevice comprising: forming a semiconductor region; forming a gateinsulation film over the semiconductor region; forming a gate electrodeover the gate insulation film, wherein the gate electrode comprisesmetal material; forming an oxide layer over and in contact with the gateelectrode, the oxide layer being in contact with a side surface of thegate electrode; doping an impurity element into the semiconductor regionto form a low-concentration impurity region in contact with achannel-forming region; after doping, removing a portion of the gateinsulation film so as to expose at least a side surface of thesemiconductor region; after doping, activating the impurity element byheating; forming a metal film so as to cover at least the side surfaceof the semiconductor region and a top surface of the gate electrode,wherein the metal film is provided over the gate electrode with theoxide layer interposed therebetween; and forming a metal silicide layerover the semiconductor region, wherein a part of the low-concentrationimpurity region overlaps with the gate electrode.
 17. A method formanufacturing a semiconductor device according to claim 16, wherein thelow-concentration impurity region is formed by doping the impurityelement in an oblique direction with respect to a top surface of thesemiconductor region.
 18. A method for manufacturing a semiconductordevice according to claim 16, wherein the metal of the metal film istitanium.
 19. A method for manufacturing a semiconductor deviceaccording to claim 16, wherein the oxide layer in contact with the sidesurface of the gate electrode is an anodic oxide film.
 20. A method formanufacturing a semiconductor device according to claim 16, wherein theimpurity element is phosphorus.
 21. A method for manufacturing asemiconductor device according to claim 16, wherein a dose in formingthe low-concentration impurity region is from 1×10¹³ to 5×10¹⁴ cm⁻². 22.A method for manufacturing a semiconductor device according to claim 16,wherein the metal of the metal film is one of nickel and platinum.
 23. Amethod for manufacturing a semiconductor device comprising: forming asemiconductor region; forming a gate insulation film over thesemiconductor region; forming a gate electrode over the gate insulationfilm, wherein the gate electrode comprises metal material; forming anoxide layer over and in contact with the gate electrode, the oxide layerbeing in contact with a side surface of the gate electrode; first dopingan impurity element into the semiconductor region to form alow-concentration impurity region in contact with a channel-formingregion; after first doping, second doping an impurity element into thesemiconductor region to form a high-concentration impurity region incontact with the low-concentration impurity region; after second doping,removing a portion of the gate insulation film so as to expose at leasta side surface of the semiconductor region; forming a metal film so asto cover at least the side surface of the semiconductor region and a topsurface of the gate electrode, wherein the metal film is provided overthe gate electrode with the oxide layer interposed therebetween; andforming a metal silicide layer over the semiconductor region.
 24. Amethod for manufacturing a semiconductor device according to claim 23,wherein the low-concentration impurity region is formed by doping theimpurity element in an oblique direction with respect to a top surfaceof the semiconductor region.
 25. A method for manufacturing asemiconductor device according to claim 23, wherein the metal of themetal film is titanium.
 26. A method for manufacturing a semiconductordevice according to claim 23, wherein the oxide layer in contact withthe side surface of the gate electrode is an anodic oxide film.
 27. Amethod for manufacturing a semiconductor device according to claim 23,wherein the impurity element is phosphorus.
 28. A method formanufacturing a semiconductor device according to claim 23, wherein adose in forming the low-concentration impurity region is from 1×10¹³ to5×10¹⁴ cm⁻².
 29. A method for manufacturing a semiconductor deviceaccording to claim 23, wherein a dose in forming the high-concentrationimpurity region is from 1×10¹⁴ to 5×10¹⁵ cm⁻².
 30. A method formanufacturing a semiconductor device according to claim 23, wherein themetal of the metal film is one of nickel and platinum.